The present invention relates to plasma display panels used for such apparatuses as color display devices for televisions.
In recent years of increasing needs for high quality, large screen televisions such as high-definition televisions, products such as a CRT, a liquid-crystal display (hereinafter referred to as a LCD), or a plasma display panel (hereinafter referred to as a PDP) are being developed to meet needs in various fields.
CRTs, popular conventional displays for televisions, have advantages in resolution, and image quality. They are, however, not ideal for screens of 40 inches or more, since they are destined to enlarge their depth and weight when the screens become large. LCDs, on the other hand, have advantages in that they consume little electricity and their driving voltage is low. However, they have technical problems for creating large screens.
PDPs, on the contrary, make it possible to create large screens with small depth. Indeed, PDP products with 50-inch diagonal screen, have been already commercialized.
PDPs can broadly be divided into direct current types (DC types) and alternating current types (AC types). These days, AC types have become mainstream, since they fit the need for upsizing screens.
A conventional AC type PDP that displays color by RGB is provided with a front cover plate and a back plate placed in a parallel direction to each other, but without contact. The inner surface of the front cover plate is provided with pairs of display electrodes placed in a stripe pattern, which is covered with a dielectric layer made of lead glass. On the inner surface of the back plate, address electrodes in a stripe pattern are placed in a direction at right angles to the display electrodes, and ribs are placed between each strip of address electrodes. Each gap developed between stripes of ribs, is provided with red, green, and blue ultraviolet-excited phosphor layers. In each discharge space surrounded by the front cover plate, the back plate, and the ribs, a discharge gas is filled.
Either a mixture of Helium (He) and xenon (Xe), or a mixture of Neon (Ne) and Xe is commonly used as a discharge gas. Pressure at which the discharge gas is filled is within the range of 100-500 Torr (which is approximately 10-70 KPa), for a discharge of 250 V or less (Please consult with M. Nobrio, T. Yoshioka, Y. Sano, K. Nunomura, SID94xe2x80x2 Digest 727-730, 1994 for details).
The light emitting principle of PDPs is basically the same as that of fluorescent lamps; it is required to apply voltage to the display electrodes to emit a normal glow discharge first, thereby making Xe emit an ultraviolet light (i.e. a xenon resonance line with a wavelength of 147 nm). This ultraviolet light, in turn, excites phosphors to emit light. However, due to the inefficiency in both the conversion rate from discharge energy to ultraviolet light and from phosphors to visible light, it is difficult for PDPs to obtain as high luminance as fluorescent lamps.
Relating to the above point, Applied Physics Vol. 51, No. 3, 1982, pp. 344-347 specifically describes that for PDPs with gas compositions of Hexe2x80x94Xe, or Nexe2x80x94Xe, the percentage of the supplied electric energy converted to ultraviolet light is about 2%, and it states further that only 0.2% of the electric energy is converted to visible light. (Also consult with Optical Technology Contact Vol. 34, No. 1, 1996, pp.25; FLAT PANEL DISPLAY 1996, Part5-3; and NHK Technology Research Vol. 31, No. 1, 1979, pp. 18)
Under such circumstances of PDPs, a technology for achieving higher luminance than the current standards is desired.
Specifically, PDPs widely used today for 40-42 inch class televisions have panel efficiency of about 1.2 1 m/w and screen luminance of 400 cd/m2 for NTSC picture element level (i.e. 640*480 pixels, cell pitch of 0.43 mm*1.29 mm, and cell size of 0.55 mm2) (consult with FLAT-PANEL DISPLAY 1997 Part 5-1 PP.198 for a detailed description). It is desired, however, to improve the stated current standards for the panel efficiency and the screen luminance to 3-5 1 m/w and 500 cd/m2, which are CRTs"" average.
Just as the demand for the improved luminance, an improved resolution level has also been an important issue in the field of PDP displays. It is possible to improve a resolution level for PDPs by shortening a pitch of the ribs and by reducing the distance between electrodes. Generally in PDPs however, the finer the resolution level, the less luminance due to the resulting smaller light emitting area. Thus, it is desirable to improve the luminous efficiency for the enhancement in luminance and to lessen the discharge voltage, as a resolution level becomes higher.
Concretely, full-spec high-definition televisions of 42 inch class, which are receiving attention these days, have 1920*1125 pixels and cell pitch of 0.15 mm*0.48 mm. In such televisions, the cell size is 0.072 mm2, which is {fraction (1/7)} to xe2x85x9 that of NTSC televisions. As already mentioned, the smaller the cell size is, the amount of light emission is destined to be smaller. Therefore, if PDPs for high-definition televisions of a 42-inch diagonal screen are to be made with conventional cell structures, the luminous efficiency and the luminance are expected to be reduced to about 0.15-0.17 1 m/w and 50-60 cd/m2, respectively.
For such PDPs to achieve the same luminance level as CRTs conforming to the NTSC standard, which is about 500 cd/m2, it is required to increase luminous efficiency by 10 times or more (i.e. 5 1 m/w or more)(consult with FLATPANEL DISPLAY 1997 Part 5-1, pp. 200 for details).
The object of the present invention is to greatly improve PDPs in luminance and luminous efficiency, compared to conventional alternating current type surface-discharge PDPs.
To achieve the object, the dielectric layer is made by laminating at least two different dielectric materials, and the panel structure is set such that an electric field with an equivalent field strength of at least 37V/cmxc2x7KPa is generated in a discharge space, when a discharge sustaining voltage is applied between pairs of display electrodes in order to selectively glow-discharge in discharge spaces in which the electric charge has been accumulated on the dielectric layer.
Note that, in this alternating current type surface-discharge PDP, field strength differs from area to area in a discharge space. What is meant here is that at least 37V/cmxc2x7KPa must be satisfied in the area of the largest field strength in a discharge space.
Here, the discharge sustaining voltage is one of those kinds that discharge only in discharge spaces accumulated with stored charge and not elsewhere. That is, it is lower voltage than discharge starting voltage which discharge in every type of discharge space.
PDPs with the above panel structures realize much enhanced panel luminance and luminous efficiency than conventional PDPs, since these panel structures enable to emit at least 37V/cmxc2x7Kpa of equivalent field strength which is much stronger than conventional PDPs.
The occurrence of this much strong electric field generates high energy electrons and a xenon excimer (molecular beam) with a wavelength of 173 nm in the discharge field, whereas conventional PDPs generate ultraviolet light mainly consisting of a xenon resonance line of 147 nm wavelength. The xenon molecular beam has phosphors toward ultraviolet light than the xenon resonance line.
The following are the factors that affect the strength of the electric field in the discharge space: an amount of xenon filled in the discharge space; a thickness and a permittivity of the dielectric layers; and a distance between a pair of display electrodes. Adequate adjustment of these factors is the key for the realization of high equivalent field strength as at least 37V/cmxc2x7KPa. Concretely, each of these factors should be adjusted as follows and that all these conditions should be combined to produce effect.
As for the amount of xenon contained in the discharge space, it should be maintained 5% or more of the total discharge gas. The enclosing pressure of the xenon should be bigger than that for conventional PDPs; specifically the desirable range is between 66.5 KPa and 200 KPa.
The thickness of the dielectric layers should be kept within the range of 3-35 xcexcm, which is thinner than conventional ones. The dielectric layers mentioned here are those formed on the opposing surfaces of pairs of display electrodes against each other.
It is true that the thinner the dielectric layers, the more the resultant effect is expected. In reality, however, it is desirable to keep them 10 xcexcm or more, taking into consideration the withstand voltage.
The permittivity of the dielectric layers should be set in the range of 6-11, to have a desirable result, which is smaller than conventional permittivity which is around 11-13. The permittivity, however, should be kept in the range of 6-9, for the display electrodes comprising such metal electrodes as Ag or Crxe2x80x94Cuxe2x80x94Cr.
This is due to the fact that as the permittivity of the dielectric layers become low, the smaller the electric capacity of the panel is, when the PDPs are assumed to be condensers. Roughly speaking, electric consumption in the driving circuit is indirectly proportional to the electric capacity of the panel. Therefore, the lower the electric capacity, the lower the electric consumption in the driving circuit.
Especially in the above case where the dielectric layers are kept thin as 35 xcexcm or less, the electric capacity of the panel tends to be larger. Thus, keeping the permittivity small (i.e. in the range of 6-11) is important in order to maintain the electric capacity of the panel small enough.
In order to fulfill the low permittivity, having two or more dielectric layers is an effective solution. For these multiple layers, it is easy to set permittivity of the dielectric layers depending on the thickness or the materials for each layer. Therefore it becomes easier than one layer to arrange the permittivity to be around 6-11 or 6-9 as desired.
As for the distance between pairs of electrodes, it is desirable to keep it in the range of 20-90 xcexcm where they face the discharge space.
It is also effective to have forms of a pair of display electrodes asymmetric to each other, or to have protrusions on at least one piece of a pair of display electrodes. The mentioned methods will increase emission of ultraviolet light by strengthening the electric field, by which the luminance and the luminous efficiency will be accordingly enhanced.